An optical encoder module typically includes a light emitting device or emitter, and a photo-detector device or detector. The emitter and the detector are spaced apart from each other and aligned to each other such that a code wheel attached to a shaft can be rotated in a gap between the emitter and the detector. The code wheel has alternating opaque and transparent areas such that light emitted by the emitter is either blocked or passed by the code wheel and then detected by the detector. An optical encoder module of this type is described in more detail in U.S. Pat. No. 4,691,101. The design of such an encoder module is also illustrated in U.S. Design Pat. No. Des. 329,193.
FIG. 1 illustrates a known optical encoder module 1 having a housing 2, and an emitter (i.e., 9, 11) and a detector 7 that face each other in an aligned relationship. The emitter (i.e., 9, and 11) and the detector 7 are spaced apart to allow a code wheel 3 to be rotated in the gap formed therebetween. The emitter (9, 11) consists of a light emitting diode (LED) 9 and a separate lens positioned over the LED 9 to receive light from the LED 9 and to produce a beam of light 17 in the direction of the detector 7. The LED 9 consists of a semiconductor die of, for example, the GaP or GaAsP type, mounted and electrically coupled via direct or wire bonding to a metal lead-frame 12. Additionally, the LED 9 may be encapsulated with a transparent epoxy material. The separate collimating lens 11 is molded from a plastics material and is mounted onto the housing 2 in correct alignment with the LED 9.
The detector 7 includes four elongated photo-detecting elements that are arranged side-by-side at different angular positions relative to the code wheel 3 so as to receive light from different portions of the light beam 17. In this way, the encoder module is able to detect both speed and direction of rotation of the code wheel 3.
As is known, the generation of a collimated beam of light from the emitter (i.e., 9, 11) to the detector 7 is an important feature that ensures accurate operation of the encoder. Light straying from the collimated beam is undesirable and contributes to parallax errors in the encoder and degradation of the operational dynamic range of the photo-detecting elements.
However, the foregoing design of encoder module suffers from various limitations in generating a collimated beam of light from the emitter (i.e., 9, 11) to the detector 7. First, the collimating lens 11 receives light from the LED 9 through a bottom planar surface and focuses light as it exits from the curved top surface. Both the top and bottom surfaces are refracting surfaces that contribute to the collimation of light from LED 9. Consequently, the ability of the collimating lens to accurately collimate light is limited not just by manufacturing tolerances and inaccuracies in producing the top surface and the bottom surface, but also by the relative positions and orientations of the top and bottom surfaces.
The position of the collimating lens 11 relative to the LED 9 is also critical to the performance of the emitter (i.e., 9, 11) in generating a collimated beam of light. Accurate placement of the lens over the LED 9 is a time consuming process that adds additional cost to the assembly of the encoder module. In addition, inclusion of a collimating lens mounted on the encoder housing limits the ability of manufacturers to design smaller encoders.